In phase-locked loops (PLLs) and other applications, it is sometimes necessary to provide clock signals, or other timing signals, of different frequencies. For applications in which power is an issue, using a high frequency clock that is a multiple of a certain desired clock frequency would be either too power intensive or not even possible using modern integrated circuit (IC) technology.
By way of example, consider the case of generating a desired 36.864 megahertz (MHz) clock from a 30 MHz reference clock. One way to achieve this is to use a PLL having a phase detector and voltage-controlled oscillator (VCO) where the 30 MHz reference clock is divided by 625 to obtain a 48 kilohertz (KHz) phase detector rate. The output of the VCO could then be multiplied by 768 to obtain the desired 36.864 MHz clock. However, this approach would result in poor jitter performance. Moreover, using a conventional PLL, a large loop filter is required or otherwise external components are needed to implement the loop filter, either of which could be costly.
The phase detector rate can be increased, thereby improving jitter performance of the PLL, by increasing the VCO output frequency to 184.32 MHz, five times the desired clock rate of 36.864 MHz. The 30 MHz reference clock is divided by 125 to obtain a 240 KHz phase detector rate. The output of the VCO would still need to be multiplied by 768 to obtain the 184.32 MHz clock. However, while jitter performance would be improved compared to the PLL arrangement in the previous example, power consumption would be increased since the VCO must run five times faster. To further increase the phase detector rate, the VCO could be potentially run at 921.6 MHz, with a divide value of 20 to obtain the desired 36.864 MHz clock rate. This would allow a phase detector rate of 1.2 MHz. Unfortunately, however, this would undesirably increase power consumption and, in some slower technologies, may not even be possible.
There exists a need, therefore, for an improved signal generator circuit that does not suffer from one or more of the problems exhibited by conventional signal generator arrangements.